Elastic anisotropy of D″ predicted from global models of mantle flow

Walker, Andrew; Forte, A. M.; Wookey, James; Nowacki, Andy; Kendall, J.‐M.

doi:10.1029/2011gc003732

Cited by 64 publications

(117 citation statements)

References 74 publications

(145 reference statements)

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“…This is expected to be far stronger than exists in reality, because at the CMB diffusion and dynamic recrystallisation are expected to operate to limit texture strength. It arises in the models because no texture-limiting condition was imposed by Walker et al (2011). Observations and previous, ray-based modelling of D ′′ also suggest that maximum shear wave anisotropy is unlikely to be as large as this.…”

Section: Isotropymentioning

confidence: 99%

“…Hence these models provide a test of forward methodologies which might better represent the elasticity in D ′′ as experienced by ScS waves which are observed. The paths were previously modelled using ray methods by Nowacki et al (2013), but for a model of ppv in D ′′ with multiple nodes radially as well as laterally; we use the Walker et al (2011) constants here to minimise the spatial variability to two dimensions. These paths sample the full range of anisotropy strengths in the models, and so are a good test of the difference in the methods.…”

Section: Geodynamically-derived Modelmentioning

confidence: 99%

“…In brief, the elastic constants were created by Walker et al (2011) using the flow field of the TX2008 model of Simmons et al (2009) to produce strain histories at each point on a 5 • -by-5 • longitudelatitude grid, 100 km above the CMB. They used estimates of postperovskite (ppv) deformation mechanisms, to perform viscoplastic self-consistent modelling along these strain histories, yielding the texture of a wholly-ppv lowermost mantle.…”

Section: Geodynamically-derived Modelmentioning

confidence: 99%

“…Although it is not expected to affect our splitting results, we choose to minimise the SdS phase here because the triplication is not the focus of this study. The elastic constants from Walker et al (2011) are mapped onto the SEM mesh with the following procedure. The coordinates (radius r, longitude and latitude) of each mesh point were evaluated, and if that point was above the top of the D ′′ model (r ≥ rCMB + h + s, where rCMB is the radius of the CMB), the elastic constants representing ak135 were returned.…”

Section: Geodynamically-derived Modelmentioning

confidence: 99%

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The limits of ray theory when measuring shear wave splitting in the lowermost mantle withScSwaves

Nowacki¹,

Wookey

2016

Geophys. J. Int.

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SUMMARYObservations of shear wave splitting provide unambiguous evidence of the presence of anisotropy in the Earth's lowermost mantle, a region known as D ′′ . Much recent work has attempted to use these observations to place constraints on strain above the core-mantle boundary (CMB), as this may help map flow throughout the mantle. Previously, this interpretation has relied on the assumption that waves can be modelled as infinite-frequency rays, or that the Earth is radially symmetric. Due to computational constraints it has not been possible to test these approximations until now. We use fully three-dimensional, generally-anisotropic simulations of ScS waves at the frequencies of the observations to show that ray methods are sometimes inadequate to interpret the signals seen. We test simple, uniform models, and for a D ′′ layer as thin as 50 km, significant splitting may be produced, and we find that recovered fast orientations usually reflect the imposed fast orientation above the CMB. Ray theory in these cases provides useful results, though there are occasional, notable differences between forward methods. Isotropic models do not generate apparent splitting. We also test more complex models, including ones based on our current understanding of mineral plasticity and elasticity in D ′′ . The results show that variations of anisotropy over even several hundred kilometres cause the ray-theoretical and finite-frequency calculations to differ greatly. Importantly, models with extreme mineral alignment in D ′′ yield splitting times not dissimilar to observations (δt ≤ 3 s), suggesting that anisotropy in the lowermost mantle is probably much stronger than previously thought-potentially ∼10 % shear wave anisotropy or more. We show that if the base of the mantle is as complicated as we believe, future studies of lowermost mantle anisotropy will have to incorporate finite-frequency effects to fully interpret observations of shear wave splitting.

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Section: Isotropymentioning

confidence: 99%

Section: Geodynamically-derived Modelmentioning

confidence: 99%

Section: Geodynamically-derived Modelmentioning

confidence: 99%

Section: Geodynamically-derived Modelmentioning

confidence: 99%

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The limits of ray theory when measuring shear wave splitting in the lowermost mantle withScSwaves

Nowacki¹,

Wookey

2016

Geophys. J. Int.

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“…Also, structures above the core-mantle boundary (CMB) such as ultra low velocity zones (ULVZ) and large low shear velocity provinces (LLSVP) are expected to be anisotropic (e.g. Panning & Romanowicz 2004;Long 2009;Nowacki et al 2011;Walker et al 2011;Cottaar & Romanowicz 2013). In the inner core, hexagonal anisotropy with a fast axis in north-south direction is observed both with normal modes (Deuss et al 2010) and inner core body waves (Irving & Deuss 2011).…”

Section: Introductionmentioning

confidence: 99%

Seismic wave propagation in fully anisotropic axisymmetric media

Driel

Nissen‐Meyer

2014

Geophysical Journal International

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S U M M A R YWe present a numerical method to compute 3-D elastic waves in fully anisotropic axisymmetric media. This method is based on a decomposition of the wave equation into a series of uncoupled 2-D equations for which the dependence of the wavefield on the azimuth can be solved analytically. Four independent equations up to quadrupole order appear as solutions for moment-tensor sources located on the symmetry axis while single forces can be accommodated by two separate solutions up to dipole order. This decomposition gives rise to an efficient solution of the 3-D wave equation in a 2-D axisymmetric medium. First, we prove the validity of the decomposition of the wavefield in the presence of general anisotropy. Then we use it to derive the reduced 2-D equations of motions and discretize them using the spectral element method. Finally, we benchmark the numerical implementation for global wave propagation at 1 Hz and consider inner core anisotropy as an application for high-frequency wave propagation in anisotropic media at frequencies up to 2 Hz.

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Development of texture and seismic anisotropy during the onset of subduction

Leo

Walker

et al. 2014

Geochem Geophys Geosyst

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[1] How reliable are shear wave splitting measurements as a means of determining mantle flow direction? This remains a topic of debate, especially in the context of subduction. The answer hinges on whether our current understanding of mineral physics provides enough to accurately translate between seismic observations and mantle deformation. Here, we present an integrated model to simulate strainhistory-dependent texture development and estimate resulting shear wave splitting in subduction environments. We do this for a mantle flow model that, in its geometry, approximates the double-sided Molucca Sea subduction system in Eastern Indonesia. We test a single-sided and a double-sided subduction case. Results are compared to recent splitting measurements of this region by Di Leo et al. (2012a). The setting lends itself as a case study, because it is fairly young and, therefore, early textures from the slab's descent from the near surface to the bottom of the mantle transition zone-which we simulate in our models-have not yet been overprinted by subsequent continuous steady state flow. Second, it allows us to test the significance of the double-sided geometry, i.e., the need for a rear barrier to achieve trench-parallel subslab mantle flow. We demonstrate that although a barrier amplifies trenchparallel subslab anisotropy due to mantle flow, it is not necessary to produce trench-parallel fast directions per se. In a simple model of A-type olivine lattice-preferred orientation and one-sided subduction, trench-parallel fast directions are produced by a combination of simple shear and extension through compression and pure shear in the subslab mantle.

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Elastic anisotropy of D″ predicted from global models of mantle flow

Cited by 64 publications

References 74 publications

The limits of ray theory when measuring shear wave splitting in the lowermost mantle withScSwaves

The limits of ray theory when measuring shear wave splitting in the lowermost mantle withScSwaves

Seismic wave propagation in fully anisotropic axisymmetric media

Development of texture and seismic anisotropy during the onset of subduction

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